Emergence of Order in Origin-of-Life Scenarios on Mineral Surfaces: Polyglycine Chains on Silica

Abstract

The polymerization of amino acids (AAs) to peptides on oxide surfaces has attracted interest owing to its high importance in biotechnology, prebiotic chemistry, and origin of life theories. However, its mechanism is still poorly understood. We tried to elucidate the reactivity of glycine (Gly) from the vapor phase on the surface of amorphous silica under controlled atmosphere at 160 °C. Infrared (IR) spectroscopy reveals that Gly functionalizes the silica surface through the formation of ester species, which represent, together with the weakly interacting silanols, crucial elements for monomers activation and polymerization. Once activated, β-turns start to form as initiators for the growth of long linear polypeptides (poly-Gly) chains, which elongate into ordered structures containing both β-sheet and helical conformations. The work also points to the role of water vapor in the formation of further self-assembled β-sheet structures that are highly resistant to hydrolysis.

Polypeptide Chain Growth Mechanisms and Secondary Structure Formation in Glycine Gas-Phase Deposition on Silica Surfaces

“…From the first CVD cycle, the amide I (1700–1600 cm –1 ) and amide II (1570–1470 cm –1 ) bands are observed, corresponding to ν CO /δ NH and δ NH /ν CN vibrations, respectively. The high intensity ratio of amide II to amide I is an indication of the formation of linear peptide chains instead of cyclic ones, while the band at 1760–1750 cm –1 may be assigned to ester groups formed between the peptide chains and surface silanols . Each step of glycine CVD causes an increase of the ester groups (positive signal in the difference spectra of Figure A,B) together with a decrease of the nearly free silanols (NFS) associated with the band at 3743 cm –1 (negative signal in the difference spectra of Figures A′ and B′): both ester and NFS represent crucial elements for monomers activation and polymerization .…”

“…In the origins of life field, template-directed polymerization has long been tested for growing RNA chains, but it is more surprising to find it at work for polypeptide chains. Note that if this interpretation is correct, the promoting effect on polyGly chain growth at this stage would only be indirectly due to the silica surface, as opposed to the direct implication of surface groups observed at the beginning of CVD …”

“…Additional graphs about the evolution of the integrated area of amide I as a function of time during Gly deposition by CVD before and after the HF cycles on the two samples: G TFHF /AX rt and G TFHF /AX 160 ; IR spectra of two bare silica samples before any Gly deposition when outgassed at rt (AX rt ) or when subjected to wetting/drying cycles then outgassed at rt (AX WD ); IR difference spectra after Gly CVD at 160 °C for 20 h on the two samples: G TF /AX rt , Gly deposition on silica outgassed at rt and G TF /AX WD , Gly deposition on silica subjected to wetting/drying cycles then sed at rt; XRD patterns for G TF /AX WD after (a) 12 C-Gly deposited on AX WD for 20 h by CVD then subjected to wetting/drying cycles and outgassing, and (b) after 15 N-Gly deposited on the same sample as a subsequent set by CVD for 15 h; evolution of the integrated area of the different types of secondary structures and ester as a function of time during 13…”

“…4,11 A previous paper has dealt with the initial steps of glycine polymerization on the silica surface upon deposition from the gas phase. 12 In the present work, we aim to study the effect of different environmental conditions on the extent and rate of the polymerization reaction and especially on the self-assembly of three systems composed of amorphous silica (AX), a model of one of the most common mineral surfaces available on primitive Earth and other extraterrestrial bodies, glycine (Gly), the most prevalent amino acid in the earliest proteins, and water, the predominant solvent on the early Earth's surface. Namely, the three systems were subjected separately to (a) temperature fluctuations (TF) during chemical vapor deposition (CVD), (b) both TF and humidity fluctuations (HF) cycles, and (c) TF and HF cycles for an extended time of CVD (35 h) while using a silica surface subjected to conditions of high water activity before the start of the polymerization.…”

“…Note that if this interpretation is correct, the promoting effect on polyGly chain growth at this stage would only be indirectly due to the silica surface, as opposed to the direct implication of surface groups observed at the beginning of CVD. 12 Structural Dynamics of the Peptide Chains Revealed by H/D Exchange. After Gly CVD for 20 h, both G TF /AX 160 and G TFHF /AX 160 were exposed to HF cycles followed by D 2 O vapor exposure until invariance of in-situ IR spectra.…”